A recombinant mannose-6-phosphate isomerase from Geobacillus thermodenitrificans (GTMpi) isomerizes aldose substrates possessing hydroxyl groups oriented in the same direction at the C2 and C3 positions such as the D- and L-forms of ribose, lyxose, talose, mannose, and allose. The activity of GTMpi for D-lyxose isomerization was optimal at pH 7.0, 70 degrees C and 1 mM Co(2+). Under these conditions, the k(cat) and K(m) values were 74,300 s(-1) and 390 mM for D-lyxose and 28,800 s(-1) and 470 mM for L-ribose, respectively. The half-lives of the enzyme at 60, 65, and 70 degrees C were 388, 73, and 27 h, respectively. GTMpi catalyzed the conversion of D-lyxose to D-xylulose with a 38% conversion yield after 3 h, and converted L-ribose to L-ribulose with a 29% conversion yield.